The present invention relates to an engine exhaust purification device provided with a catalyst and more specifically to a control arrangement for an exhaust gas purification device which maintains the air-fuel ratio in a catalytic converter at stoichiometric based on specific oxygen adsorption/release characteristics of the catalyst.
In order to remove hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) from engine exhaust gas by using a three-way catalyst, it is important to maintain the gaseous environment of the catalyst to have an oxygen concentration that corresponds closely to a stoichiometric air-fuel ratio of the fuel mixture provided to the engine.
In order to achieve this end, it has been proposed to provide a catalytic converter with the capacity of storing and releasing oxygen in response to the current oxygen concentration so that the gaseous environment of the catalyst is maintained in an atmosphere that has an oxygen concentration which corresponds to the stoichiometric air-fuel ratio. While precious metals which are used in the catalyst have a function of adsorbing and releasing oxygen, it has been proposed to increase the oxygen storage capacity in a manner which achieves the required level by including an oxygen absorbing material such as cerium oxide, barium or base metals on the catalyst substrate.
U.S. Pat. No. 5,842,340 issued on Dec. 1, 1998 in the name of Bush et al., discloses the above type of catalytic converter along with a calculation method for determining the current oxygen storage amount of the catalyst. This method estimates the oxygen storage amount of the catalyst by analysis of an output signal of oxygen sensors provided in the outlet and inlet of the catalytic converter. The air-fuel ratio of the fuel mixture supplied to the engine is thereby controlled so that the oxygen storage amount coincides with a target value.
A similar method is also disclosed in Tokkai Hei 5-195842 published by the Japanese Patent Office in 1993 and Tokkai Hei 7-259602 published by the Japanese Patent Office in 1995.
U.S. Pat. No. 6,116,021 issued on Sep. 12, 2000 in the name of Schumacher et al. discloses providing an estimate of the desorption capacity by integrating an expression from complete saturation to complete depletion. This reference further indicates that an inverted integral may provide a more accurate and reproducible estimate of the oxygen storage and release capacity. Nevertheless, this document does not contain any mention of setting a target storage level about which the air-fuel ratio should be adjusted.
The target value for the oxygen storage amount is determined based on the oxygen storage capacity of the catalyst as estimated from the variation in the output of the two oxygen sensors. However, when there is a deviation in the performance of the two oxygen sensors, for example, the calculated value of the oxygen storage capacity can drift and become either too large or too small and, as a result, the actual oxygen storage amount may be controlled to a value which differs from the desired target value. This type of deviation in the target value has an adverse effect on the control of the oxygen concentration in the exhaust gas. This invention is therefore directed to increasing the accuracy of the estimation of the oxygen storage capacity of the catalyst.
In order to achieve the above, this invention provides an exhaust emission control arrangement for such an engine that comprises a fuel supply mechanism and an exhaust passage wherein the control arrangement comprises a catalytic converter disposed in the exhaust gas passage The catalytic converter houses a three-way catalyst. The controller also comprises a first oxygen sensor which detects an oxygen concentration of exhaust gas upstream of the catalyst as an upstream oxygen concentration, a second oxygen sensor which detects an oxygen concentration of exhaust gas downstream of the catalyst as a downstream oxygen concentration, and a microprocessor.
The microprocessor is programmed to calculate, from the upstream oxygen concentration, an excess/deficiency oxygen concentration in exhaust gas upstream of the catalyst with respect to a stoichiometric oxygen concentration which corresponds to a stoichiometric air-fuel ratio of a fuel mixture provided to the engine, calculate an oxygen storage amount of the catalyst based on the excess/deficiency oxygen concentration, calculate a specific period oxygen storage amount of the catalyst during a period in which the upstream oxygen concentration is higher than the stoichiometric concentration while the downstream oxygen concentration is in a predetermined concentration range including the stoichiometric oxygen concentration, and calculate a specific period oxygen release amount of the catalyst during a period in which the upstream oxygen concentration is lower than the stoichiometric concentration while the downstream oxygen concentration is in the predetermined concentration range.
The microprocessor is further programmed to sample a specific period oxygen storage amount as a maximum oxygen storage amount at a time at which the downstream oxygen concentration becomes greater than the predetermined concentration range, sample a specific period oxygen release amount as a maximum oxygen release amount at a time at which the downstream oxygen concentration becomes smaller than the predetermined concentration range, and calculate an average value of the maximum oxygen storage amount and the maximum oxygen release amount.
The microprocessor is further programmed to determine a target value of the oxygen storage amount based on the average value, and control a fuel supply amount of the fuel supply mechanism to cause the oxygen storage amount of the catalyst to coincide with the target value.
More specifically, a first aspect of the invention resides in an exhaust purification arrangement for an engine, comprising: a catalyst provided in an exhaust passage of the engine; a front sensor which detects an excess oxygen concentration of oxygen flowing into the catalyst; and a microprocessor programmed to: estimate a first amount of oxygen stored in the catalyst, the first amount estimated to be stored at a first rate; estimate a second amount of oxygen stored in the catalyst; wherein the first rate is estimated based on the excess oxygen concentration and a relationship between the first amount and the second amount; and control an air/fuel ratio of the engine based on an average of the first and second amounts.
A second aspect of the invention resides in a method and apparatus for controlling an air-ratio of an engine having a catalytic converter disposed in an exhaust gas passage connected to the engine, the catalytic converter storing and releasing oxygen and having a saturated in oxygen condition and a completely oxygen depleted condition, comprising: estimating a first storage capacity of the catalytic converter by integrating an excessive oxygen flow rate entering the catalytic converter over a first time interval from the completely depleted condition to the saturated condition; estimating a second storage capacity by integrating an oxygen desorption flow rate over a second time internal from the saturated condition to the completely depleted condition; determining a target value for the oxygen storage level within the catalytic converter based on a mathematical result derived using the first storage capacity and the second storage capacity; and controlling the air-fuel ratio of the exhaust gas supplied to the catalytic converter to maintain the level of oxygen stored within the catalytic converter to maintain the determined target value.
In the above method, the step of determining a target value comprises steps of: averaging the first and second storage capacities; and applying a value, derived using the average, as the target value for the oxygen storage level.
The first time period is determined between a first point in time when the air-fuel ratio downstream of the catalytic converter switches from lean to rich and a second point in time when the air-fuel ratio switches from rich to lean, and wherein the second time period is determined between a third point in time wherein the air-fuel ratio switches from rich to lean and a fourth point in time when the air-fuel ratio switches from lean to rich.
Another aspect of the invention resides in a method and apparatus for controlling the air-fuel ratio of an atmosphere in an catalytic converter which is operatively connected with an internal combustion engine, comprising: storing oxygen on a first material in the catalytic converter which adsorbs and releases oxygen rapidly; storing oxygen on a second material in the catalytic converter which adsorbs and releases oxygen more slowly than the first material; and controlling the air-fuel ratio of the exhaust gas entering the catalytic converter to control the amount of oxygen which is adsorbed on the first material to a predicted predetermined amount which is less than the maximum amount of oxygen which can be adsorbed onto the first material; checking the predicted predetermined amount by controlling the air-fuel ratio of the exhaust gas supplied into the catalytic converter so that the first material becomes saturated with oxygen; controlling the amount of oxygen in the exhaust gas supplied to the catalytic converter so that oxygen is released from the first material; detecting a first period required for the oxygen storage status of the first material to go from full saturation to total depletion; controlling the amount of oxygen in the exhaust gas supplied to the catalytic converter so that oxygen is adsorbed onto the first material; detecting a second period required for the oxygen storage status to go from total depletion to full saturation; detecting the respective mass flow amount of oxygen during the first and second periods and determining the amount of oxygen released and the amount of oxygen adsorbed; comparing the first and second amounts; and determining a target oxygen storage value based on the comparison of the first and second amounts.
Yet another aspect of the present invention resides in an exhaust emission control arrangement for an engine, the engine comprising a fuel supply mechanism and an exhaust passage, the controller comprising: a catalytic converter disposed in the exhaust gas passage, the catalytic converter storing a three-way catalyst; means for detecting an oxygen concentration of exhaust gas upstream of the catalyst as an upstream oxygen concentration; means for detecting an oxygen concentration of exhaust gas downstream of the catalyst as a downstream oxygen concentration; means for calculating, from the upstream oxygen concentration, an excess/deficiency oxygen concentration in exhaust gas upstream of the catalyst with respect to a stoichiometric oxygen concentration which corresponds to a stoichiometric air-fuel ratio of a fuel mixture provided to the engine; means for calculating an oxygen storage amount of the catalyst based on the excess/deficiency oxygen concentration; means for calculating a specific period oxygen storage amount of the catalyst during a period in which the upstream oxygen concentration is higher than the stoichiometric concentration while the downstream oxygen concentration is in a predetermined concentration range including the stoichiometric oxygen concentration; means for calculating a specific period oxygen release amount of the catalyst during a period in which the upstream oxygen concentration is lower than the stoichiometric concentration while the downstream oxygen concentration is in the predetermined concentration range; means for sampling a specific period oxygen storage amount as a maximum oxygen storage amount at a time at which the downstream oxygen concentration becomes greater than the predetermined concentration range; means for sampling a specific period oxygen release amount as a maximum oxygen release amount at a time at which the downstream oxygen concentration becomes smaller than the predetermined concentration range; means for calculating an average value of the maximum oxygen storage amount and the maximum oxygen release amount; means for determining a target value of the oxygen storage amount based on the average value; and means for controlling a fuel supply amount of the fuel supply mechanism to cause the oxygen storage amount of the catalyst to coincide with the target value.
Another aspect of the invention resides in a method of determining a target oxygen content of a catalyst forming part of an exhaust emission control for an internal combustion engine comprising the steps of: determining a first maximum amount of oxygen that a catalyst material can rapidly adsorb; determining a second maximum amount of oxygen that the catalyst material can rapidly release; and averaging the first and second maximum amounts and setting the target oxygen content on a predetermined percentage of the average.
This technique also determines a third maximum amount of oxygen that a storage material associated with the catalyst can store; and determines the effect of the oxygen stored in the storage material on the release characteristics of the catalyst material.
A still further aspect of the invention resides in arrangement for determining a target oxygen content of a catalyst forming part of an exhaust emission control for an internal combustion engine, comprising: an apparatus including and air-fuel ratio sensor and a gas flow sensing arrangement for: determining a first maximum amount of oxygen that a catalyst material can rapidly adsorb; determining a second maximum amount of oxygen that the catalyst material can rapidly release; and averaging the first and second maximum amounts and setting the target oxygen content on a predetermined percentage of the average.
If not specifically set forth above, the invention of course includes apparatus/arrangements for carrying out the steps enumerated in connection with the disclosed techniques.
According to the invention described above, the accuracy of the estimation of the oxygen storage capacity of the catalyst is increased, and accurate control of the gaseous environment of the catalyst is realized. The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.